Remarkable progress has occurred recently in miniaturization and weight reduction of electronic devices. Consequently, greater demands have been placed on the reduction of size and weight of batteries used as power supplies for driving such electronic devices. To meet such demands for miniaturization and weight reduction, nonaqueous electrolyte secondary batteries represented by lithium ion secondary batteries have been developed as electrical storage devices. Further, electric double layer capacitors are known as electrical storage devices exhibiting excellent characteristics such as high output density and good cycle performance. Furthermore, attentions have been drawn to lithium ion capacitors based on a combination of electrical storage principles of lithium ion secondary batteries and electric double layer capacitors. Lithium ion capacitors are electrical storage devices that comply with applications requiring high energy density characteristics and high output characteristics.
Various studies have been carried out on negative electrode materials used in such electrical storage devices. For example, Patent Literature 1 discloses that an electrical storage device with high energy, high output and excellent low-temperature characteristics is obtained by controlling the mesopore volume of a carbon material used as a negative electrode active material. Further, Patent Literature 2 discloses that a nonaqueous secondary battery with high resistance to the decomposition of electrolytic solution and excellent charge/discharge efficiency is obtained by using carbon powder as a negative electrode material which has a ratio V2/V1 of not less than 1 wherein V1 is the volume of pores with pore diameters of 2 to 50 nm and V2 is the volume of pores with pore diameters of 50 to 200 nm.